Standard radiation therapy is often limited by normal shifts within the human anatomy. For example, tissues and organs can settle differently each time a patient climbs on a treatment table. Weight fluctuations and normal breathing can also change a tumor's locale. Radiation oncologists have traditionally compensated for tumor movement by enlarging treatment areas, exposing more healthy tissue to the cell-killing effects of radiation. Doses are typically lowered to avoid complications.
New techniques are are now available that allow radiation therapy specialist to locate and target tumors with unprecedented accuracy. This next generation treatment technology — called image-guided radiation therapy (IGRT) — represents a major step in the fight against cancer. IGRT allows for the use of higher, more effective doses of radiation because there is less risk of damaging nearby healthy tissue.
How It Works
When patients are positioned on a treatment couch, an X-ray system mounted on a robotic arm is rotated around their body to gather images that help pinpoint a tumor's exact location. These images are then compared with existing images (MRI, CT, etc.) to determine if the tumor has moved since the last treatment, which is a normal occurrence.
For Dynamic Targeting IGRT, the linear accelerator is outfitted with a number of sophisticated imaging devices that provide the clinician with images that help to guide the treatment.
Central to this approach is Varian's On-Board Imager® kV imaging system, an imaging tool that is attached to the treatment machine on a pair of robotic arms, and produces low-dose, high-resolution kilovoltage X-ray images for pinpointing the position of the tumor immediately prior to treatment. The On-Board Imager can be operated in three distinct imaging modes to generate different types of images, including:
- Radiographic (two-dimensional)
- Fluoroscopic (moving, in real-time)
- Cone-beam CT (three-dimensional)
These distinct images types provide doctors with different information about the tumor and surrounding anatomy and can reveal changes in tumor shape, size or position over a multi-week course of treatment. Fluoroscopic images can be used to track tumor motion for a clear indication of exactly how a tumor will move during treatment due to respiration or other normal physiological processes. This enables doctors to design optimal treatments for their patients.